The environmental protection authorities are placing ever more stringent demands on the pulp industry to decrease the use of chlorine gas in bleaching. Permitted discharges of organic chlorine compounds (AOX) together with the effluent water from the bleaching plant have gradually been lowered and are now at such a low level that pulp mills have in many cases stopped using chlorine gas. Instead, chlorine dioxide is used as the sole bleaching agent. For the same bleaching effect, chlorine dioxide forms lower quantities of AOX than does chlorine gas. However, the use of chlorine dioxide has also been called into question. On the one hand, the environmental protection authorities in certain countries demand that the discharges of organic chlorine compounds should be reduced to such a low level that it is scarcely possible to meet the requirements even when using only chlorine dioxide for bleaching. On the other hand, environmental movements in several countries, especially in Germany, have persuaded consumers to demand paper products which have been bleached entirely without using either chlorine gas or chlorine dioxide.
The pulp industry is therefore searching for methods which permit bleaching of pulp without using these chemicals. One such method has been developed by the Swedish company Eka, which supplies bleaching chemicals to the pulp industry. The bleaching method, which is termed LIGNOX (see SE-A-8902058), involves the unbleached pulp first being delignified with oxygen and then, after washing, being treated with EDTA or another suitable chelating agent in order to remove heavy metals bound within the pulp. After the EDTA stage (Q), there follows an intensive peroxide bleaching stage (P), i.e. hydrogen peroxide. The charge of hydrogen peroxide (H.sub.2 O.sub.2) is relatively high, being 15-35 kg per ton of pulp, depending on the required brightness and on the bleachability of the pulp. The time is quite long, being 4 hours or more, and the temperature high, being 80-90.degree. C.
However, the lignox method only provides a limited increase in brightness. The maximum brightness obtainable depends on the bleachability of the pulp and on the charge of peroxide. Brightnesses in the range of from 80-82 ISO have been recorded. Further bleaching stages over and above the peroxide stage are required in order to achieve higher degrees of brightness.
In this connection, ozone is a bleaching chemical of interest. Several experiments have shown that the use of an ozone bleaching stage (Z) results in the lignin content being decreased, i.e. in the kappa number being reduced. This is important, since a pulp bleached only with peroxide or oxygen/peroxide still contains a relatively high content of lignin, which affects the colour reversion tendencies of the pulp. On being heated or irradiated with sunlight, the pulp yellows. Further lignin is removed by ozone treatment, resulting in the brightness of the pulp becoming more stable.
Eka has shown that oxygen-bleached sulphate pulp which has been treated with EDTA in order to remove heavy metals and subsequently bleached with peroxide and ozone, in accordance with the sequence QPZ, yields brightnesses in the range from 82-87 ISO, depending on pulp type. By extending the bleaching sequence with an additional peroxide stage, and bleaching in accordance with the sequence QPZP, brightnesses in the range from 87-89 ISO can be achieved, depending on pulp type, see "Non Chlorine Bleaching", J. Basta, L. Andersson, W. Hermansson; Proceedings Mar. 2-5, 1992--Westin Resort--Hilton Head--South Carolina; Copyright by Miller Freeman Inc.
In another patent application, SE9101300 (SE-B-468355), Eka has described an additional bleaching method in which chelating agents are used prior to an ozone or peroxide stage. This application principally relates to ozone being used directly after a chelating-agent stage.
A prerequisite for achieving high degrees of brightness while consuming only moderate quantities of bleaching agent is that the pulp, prior to bleaching, should have been delignified to a low kappa number, preferably to a kappa number lower than 16. Normally, there is a deterioration in quality, above all a loss of fibre strength, if delignification in the digester house, and oxygen delignification, are taken too far. Nevertheless, in order to achieve a brightness of from 85-90% ISO, as required by the market, and acceptable strength, it is a prerequisite, in order to be able to carry out a chlorine-free bleaching process, that the pulp be produced by a disclosure process which yields a low kappa number, less than 20 and preferably less than 15, and a viscosity of at least about 1000 dm.sup.3 /kg. This process should preferably contain an oxygen-delignification stage. However, using the modified cooking methods which have been developed in recent years, it has proved possible to achieve very low kappa numbers without any loss of strength. For example, it is possible, using a modification of Kamyr's continuous cooking process MCC (modified continuous cooking) combined with MC-oxygen-delignification, to get down to, and even below, kappa numbers of 10 for soft wood and 8 for hard wood, with strength properties being retained; if use is additionally made of Kamyr's patented ITC (IsoThermal Cooking) process, even lower kappa numbers can be obtained, with kappa numbers of less than 15 after the digester easily being achieved, which, in the case of soft wood, provides kappa numbers of less than 10 after oxygen delignification.
The modification in accordance with the ITC or MCC process involves the Hi-heat washing zone in the lower part of the continuous digester also being utilized for countercurrent cooking (see EP-A-476230). This is brought about by heating to full cooking temperature in the Hi-heat circulation and by adding alkaline cooking liquid to this same circulation. The total cooking time in countercurrent is thereby extended to 3-4 hours as against about 1 hour in the case of conventional MCC. This results in a very low concentration of lignin being obtained at the end of the cooking, in turn providing improved selectivity in the delignification, i.e. the lignin in the wood is released efficiently without the cellulose being attacked to any appreciable extent. By these means, the cooking and oxygen delignification can be carried out down to very low kappa numbers without impairing the properties of the pulp, thereby ensuring that bleaching with chemicals of the peroxide, etc., type can be used for bleaching up to full brightness with the properties of the pulp at the same time remaining acceptable.
As is evident from the publications cited above, known technique indicates that a Q stage, i.e. a chelating agent, should always be used prior to a Z or a P stage, preferably with an intermediate wash.